1. Field of the Invention
The present invention relates to a tire reinforcing steel cord and a pneumatic tire using the steel cord, and more particularly to a steel cord destined to reinforce the carcass and belt of a pneumatic tire and having a reduced weight and improved durability, and a pneumatic tire using the steel cord.
2. Description of the Prior Art
These days, a best possible gas mileage of the wheeled motor vehicles has been advocated for environmental protection of the earth. To this end, such vehicles are required to be as light as possible and thus the wheel tires should also be lighter. It has been tried in this field of industry to reduce the amount of steel cord used to reinforce the tire without making the sacrifice of the tire strength by improving the strength of the steel cord. Also, however, it is well-known that increase in tensile strength per unit sectional area of steel filaments forming together a steel cord will deteriorate the resistance to fatigue of the steel cord.
To improve the tire's resistance to fatigue, some proposals have so far been made in the field of industry. A typical one of them is disclosed in the Japanese Unexamined Patent Publication No. (Heisei) 5-71084. According to this reference, a steel wire is manufactured by plating high-carbon steel filaments of which the carbon content is more than 0.6%, drawing them and using drawing dies of less than 8 degrees in approach angle at the end of the drawing process to reduce the axial residual tensile stress in the wire surface layer to less than 45 kg/mm.sup.2 as measured by the X-ray diffraction method.
However, this solution is not advantageous for the following reasons. Namely, straight steel filaments being twisted together are plastically deformed, causing a corresponding residual tensile stress in the surface layer inside the axis of the spiraled steel filaments which would result from untwisting of the steel cord, so that the steel cord thus obtained will not have so much improved resistance to corrosion fatigue.
Further, the Japanese Unexamined Patent Publication No. (Showa) 57-149578 discloses a metal wire which is said to have an improved mechanical-fatigue resistance attainable by compressing and evenly dispersing the residual stress in the outer surface layer. To improve the resistance to corrosion fatigue, this reference also proposes to use an alloy wire containing, as added thereto, an element which will impart an corrosion resistance to the wire rod for an intended steel cord, or to penetrate rubber into the steel cord in order to inhibit contact of the steel filaments with water.
However, since the steel cord is processed for a residual compressive stress to be imparted mainly to all the outer circumference of the steel cord, so the residual compressive stress will be caused mainly in the surface layer outside the spiral of the steel filaments. Thus the steel cord will not have the corrosion-fatigue resistance improved so much as expected at the depth thereof to which rubber cannot be easily penetrated. This is because no flyer processing of the steel cord is required after spiraling the steel filaments has caused at the outer surface thereof a sufficient residual compressive stress for a corrosion-fatigue resistance.
Penetration of rubber into the steel cord to prevent the steel filaments from getting in contact with water cannot effectively improve the corrosion resistance of the steel cord unless the rubber penetration is sufficient. Even if the rubber could be penetrated in a sufficient amount into the steel cord, a void or voids takes place in the boundary between the steel filaments and rubber unless the rubber is sufficiently adherent to the steel filaments. Such void or voids deteriorate the corrosion resistance of the steel cord.
On the other hand, the addition of the element which imparts a corrosion resistance to the wire rod will lead to increased manufacturing costs of the wire rod and deteriorate the elongation of the wire rod.
Furthermore, the Japanese Unexamined Patent Publication No. (Heisei) 3-104821 also discloses a method of producing a highly strong, high-ductility thin steel wire. According to this reference, a wet continuous drawing process is employed in which an increased number of drawing posts each using a die of which the taper angle is reduced to limit heat buildup in the process of drawing. However, this method is not advantageous in the low efficiency of drawing work.
Also the Japanese Unexamined Patent Publication No. (Heisei) 4-126605 discloses a method of designing a lighter tire without making the sacrifice of the tire durability, in which a steel cord made of a carbon steel containing carbon in 0.90 to 0.95% by weight and chromium in 0.10 to 0.40% by weight is used to reinforce the tire. A reduction of tire weight may be attained by using a reduced amount of wire rod for the steel rod, but there is a possibility that the tire durability may be correspondingly lower. Such selection of a carbon steel containing special ingredients will add to the manufacturing costs of the steel cord.
The conventional pneumatic tires for use with trucks and buses use as the carcass thereof a steel cord made of steel filaments each of 2,750 to 3,150N/mmn.sup.2 in tensile strength and formed in a two-layer twisted structure (3+9.times.0.23+1) or a three-layer twisted structure (3+9+1.5.times.0.175+1). However, if the steel cord is desired to have a structure of 3+9.times.0.21 in which the thinner steel cord is used to reduce the total weight of the tire, the number of steel cords encased in the carcass ply in the tire has to be increased to maintain the tire strength. As a result, the spacing between the steel cords in the carcass is too narrow, which causes the return end of the carcass ply to be broken more easily.
The tire is likely to be bruised deeply to near the steel cord by any projection on the surface or side wall of the road from time to time. Water may invade through the bruise into the tire and corrode the steel filaments, which will deteriorate the tire durability.
Further, a method also has been proposed in which the number of steel filaments forming the outer layer of a tire is reduced for a deeper penetration of rubber into the steel cord in order to prevent the steel filaments from being corroded. However, this method cannot attain any satisfactory effect unless the rubber is penetrated sufficiently deep into the steel cord. Even if the rubber is penetrated deeply, water the rubber itself contains will spoil the resistance of the steel filaments to corrosion fatigue.